Tin is found in nature primarily as the oxide in the mineral cassiterite (SnO.sub.2). This occurs in crystalline form in many tin ores, but also in fibrous habit as wood tin.
More recently, in Mexico, Australia, and Bolivia, tin oxide has been found to occur in extremely finely disseminated form, identifiable only through x-ray diffraction and by assay, and not visually or by ordinary microscopy. In several of these occurrences of very fine tin oxide, quite substantial tonnages thereof have been blocked out.
The recovery of tin after mining the cassiterite ores conventionally involves the treatment by crushing and grinding to liberate the minerals followed by gravity separation processes to concentrate the mineral in the size range from approximately 10-mesh to 325-mesh (43 microns). Conventionally this may be followed by flotation procedures for size fractions in the range from 325-mesh to approximately 5 to 10 microns in size. Flotation will have been preceded by a desliming operation in hydraulic cyclones to separate all minerals finer than 5 to 10 microns. While the purpose of this desliming is to remove those minerals such as clays and hydrated iron oxides which interfere with cassiterite flotation in the coarser sizes, all cassiterite in the finer sizes is also discarded.
For those ores in which the tin oxide is initially already in extremely fine sizes, gravity separation and flotations are ipso facto inapplicable; for such ores the many efforts to process them have hitherto been unsuccessful.
Thus, the slimes produced by gravity-flotation plants which may represent from ten to thirty percent of the original tin contents of the ores, and those ores containing the tin oxide initially in very fine disseminations, are not presently considered to be tin resources.